Ethylene oxide catalyst and process

ABSTRACT

This invention relates to an ethylene oxide catalyst having improved initial selectivity which catalyst comprises silver, a promoting amount of alkali metal, a promoting amount of rhenium and a promoting amount of rhenium-co-promoter selected from phosphorus, boron and mixtures thereof, supported on a porous refractory support.

This is a division of application Ser. No. 08/333,007, filed Nov. 1,1994 U.S. Pat. No. 5,545,603.

FIELD OF THE INVENTION

The invention relates to silver-containing catalysts suitable for thepreparation of ethylene oxide and to the use of the catalysts for thepreparation of ethylene oxide.

BACKGROUND OF THE INVENTION

Catalysts for the production of ethylene oxide from ethylene andmolecular oxygen are generally supported silver catalysts. Suchcatalysts are typically promoted with alkali metals. The use of smallamounts of the alkali metals potassium, rubidium and cesium were notedas useful promoters in supported silver catalysts in U.S. Pat. No.3,962,136, issued Jun. 8, 1976, and U.S. Pat. No. 4,010,115, issued Mar.1, 1977. The use of other co-promoters, such as rhenium, or rheniumalong with sulfur, molybdenum, tungsten and chromium is disclosed inU.S. Pat. No. 4,766,105, issued Aug. 23, 1988, and U.S. Pat. No.4,808,738, issued Feb. 28, 1989. U.S. Pat. No. 4,908,343, issued Mar.13, 1990, discloses a supported silver catalyst containing a mixture ofa cesium salt and one or more alkali metal and alkaline earth metalsalts.

U.S. Pat. No. 3,844,981 issued Oct. 29, 1974, U.S. Pat. No. 3,962,285issued Jun. 8, 1976 and British Patent No. 1,325,715, published Aug. 8,1973, disclose the use of silver-rhenium ethylene oxide catalysts. Inthese patents a high surface area silver derivative such as silver oxideis impregnated with a rhenium solution and subsequently reduced toprovide metallic rhenium alloyed with the silver. The '285 patentdiscloses the use of KOH to precipitate Ag₂ O from AgNO₃. There is nodisclosure in the patents of the use of suitable inert supports such asporous refractory supports. U.S. Pat. No. 4,548,921, issued Oct. 22,1985, discloses the use of rhenium in silver-supported ethylene oxidecatalysts. In this reference, the rhenium is first placed on the supportin the form of finely divided metal particles and the silver issubsequently deposited on the outer surface of the particles. U.S. Pat.No. 3,316,279, issued Apr. 25, 1967, discloses the use of rheniumcompounds, particularly ammonium and alkali metal perrhenates for theoxidation of olefins to olefin oxides. In this reference, however, therhenium compounds are used, unsupported, along with a reaction modifier(cyanides, pyridines or quinolines) in a liquid phase reaction. U.S.Pat. No. 3,972,829, issued Aug. 3, 1976, discloses a method fordistributing catalytically active metallic components on supports usingan impregnating solution of catalyst precursor compound and an organicthioacid or a mercaptocarboxylic acid. Catalytically active metalsinclude metals of Groups IVA, IB, VIB, VIIB and VIII, including rheniumand which may be in either the oxidized or reduced state. U.S. Pat. No.4,459,372, issued Jul. 10, 1984, discloses the use of rhenium metal incombination with a surface metallated (using Ti, Zr, Hf, V, Sb, Pb, Ta,Nb, Ge and/or Si) alumina or silica. U.S. Pat. No. 4,005,049, issuedJan. 25, 1977, teaches the preparation of a silver/transition metalcatalyst useful in oxidation reactions. In this instance, the silverserves as both a catalyst and a support for the transition metalco-catalyst. In U.S. Pat. No. 4,536,482, issued Aug. 20, 1985,catalytically active metals such as Ag and Re are co-sputtered alongwith a co-sputtered support material on a particular support. U.S. Pat.No. 4,257,976, issued Mar. 24, 1981, discloses a catalyst combination ofreduced silver, a carbonate of a rare earth metal and yttrium, a salt ofan alkali or alkaline earth metal and a catalyst carrier. U.S. Pat. No.4,766,105, issued Aug. 23, 1988, discloses adding a rhenium co-promoterselected from sulfur, molybdenum, chromium, tungsten and mixturesthereof to an alkali metal/rhenium doped supported silver catalyst.However, none of these references discloses the use of phosphorus and/orboron rhenium co-promoters on a silver-based, alkali metal/rhenium-dopedsupported catalyst and the resulting improvements in initial selectivityof the catalysts.

SUMMARY OF THE INVENTION

The invention relates to a catalyst for the production of ethylene oxidecatalyst from ethylene and molecular oxygen in the vapor phase whichcatalyst comprises a catalytically effective amount of silver, apromoting amount of alkali metal, a promoting amount of rhenium and arhenium co-promoter selected from phosphorus, boron and mixtures thereofsupported on a porous, refractory support.

It has been found that catalysts containing a phosphorus and/or-boronrhenium co-promoter have higher initial selectivities than thoseobtained with catalysts containing no rhenium co-promoter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, in the vapor phase reaction of ethylene with oxygen toproduce ethylene oxide, the ethylene is present in at least a doubleamount (on a molar basis) compared with oxygen, but frequently is oftenmuch higher. Therefore, the conversion is calculated according to themole percentage of oxygen which has been consumed in the reaction toform ethylene oxide and any oxygenated by-products. The oxygenconversion is dependent on the reaction temperature, and the reactiontemperature is a measure of the activity of the catalyst employed. Thevalue T₄₀ indicates the temperature at 40 mole percent oxygen conversionin the reactor and the value T is expressed in ° C. This temperature forany given catalyst is higher when the conversion of oxygen is higher.Moreover, this temperature is strongly dependent on the employedcatalyst and the reaction conditions. The selectivity (to ethyleneoxide) indicates the molar amount of ethylene oxide in the reactionproduct compared with the total molar amount of ethylene converted. Inthis specification, the selectivity is indicated as S₄₀, which means theselectivity at 40 mole percent oxygen conversion. The initialselectivity of a catalyst is important because less ethylene is wastedin forming undesired by-products, thus increasing the overall efficiencyand advantaging the economics of the more selective process.Additionally, increased selectivity can allow a process to operate at alower conversion to obtain the same amount of ethylene oxide, thusallowing the process to operate at a lower temperature. As used herein,"selectivity" is used to refer to the selectivity of ethylene oxidecatalysts when measured at a given constant oxygen conversion level of40% at a gas hourly space velocity of approximately 3300 and whenmeasured after the catalyst has been placed on stream for at leastseveral days.

The catalysts of the instant invention comprise a catalyticallyeffective amount of silver, a promoting amount of alkali metal, apromoting amount of rhenium and a promoting amount of a rheniumco-promoter selected from phosphorus, boron and mixtures thereof,supported on a porous, refractory support.

In general, the catalysts of the present invention are prepared byimpregnating porous refractory supports with silver ions or compound(s),complex(es) and/or salt(s) dissolved in a suitable solvent sufficient tocause deposition on the support of from about 1 to about 25 percent byweight, basis the weight of the total catalyst, of silver. Theimpregnated support is then separated from the solution and thedeposited silver compound is reduced to metallic silver. Also depositedon the support either prior to, coincidentally with, or subsequent tothe deposition of the silver will be suitable ions, or compound(s)and/or salt(s) of alkali metal dissolved in a suitable solvent. Alsodeposited on the support either prior to, coincidentally with, orsubsequent to the deposition of the silver and/or alkali metal will besuitable rhenium ions or compound(s), complex(es) and/or salt(s)dissolved in an appropriate solvent. Also deposited on the carriereither prior to, coincidentally with, or subsequent to the deposition ofthe silver and/or alkali metal and/or rhenium will be suitable ions orsalt(s), complex(es) and/or compound(s) of phosphorus and/or borondissolved in an appropriate solvent.

The carrier or support employed in these catalysts in its broadestaspects can be any of the large number of conventional, porousrefractory catalyst carriers or support materials which are consideredrelatively inert in the presence of ethylene oxidation feeds, productsand reaction conditions. Such conventional materials are known to thoseskilled in the art and may be of natural or synthetic origin andpreferably are of a macroporous structure, i.e., a structure having asurface area below about 10 m² /g and preferably below about 3 m² /g.Particularly suitable supports are those of aluminous composition.Examples of supports which have been used as supports for differentcatalysts and which could, it is believed, be used as supports forethylene oxide catalysts are the aluminum oxides (including thematerials sold under the trade name "Alundum"), charcoal, pumice,magnesia, zirconia, kieselguhr, fuller's earth, silicon carbide, porousagglomerates comprising silica and/or silicon carbide, silica, magnesia,selected clays, artificial and natural zeolites and ceramics. Refractorysupports especially useful in the preparation of catalysts in accordancewith this invention comprise the aluminous materials, in particularthose comprising alpha alumina. In the case of alpha alumina-containingsupports, preference is given to those having a specific surface area asmeasured by the B.E.T. method of from about 0.03m² /g to about 10 m² /g,preferably from about 0.05 m² /g to about 5 m² /g, more preferably fromabout 0.1 m² /g to about 3m² /g, and a water pore volume as measured byconventional water absorption techniques of from about 0.1 to about 0.75cc/g by volume. The B.E.T. method for determining specific surface areais described in detail in Brunauer, S., Emmet, P. Y. and Teller, E., J.Am. Chem. Soc., 60, 309-16 (1938).

Certain types of alpha alumina containing supports are particularlypreferred. These alpha alumina supports have relatively uniform porediameters and are more fully characterized by having B.E.T. specificsurface areas of from about 0.1 m² /g to about 3 m² /g, preferably fromabout 0.1 m² /g to about 2 m² /g, and water pore volumes of from about0.10 cc/g to about 0.55 cc/g. Typical properties of some supports foundparticularly useful in the present invention are presented in Table I.Suitable manufacturers of carriers comparable to those in Table Iinclude Norton Company and United Catalysts, Inc. (UCI).

                                      TABLE I                                     __________________________________________________________________________    Carrier       A   B   C   D   E   F                                           __________________________________________________________________________    B.E.T. Surface Area, m.sup.2 /g.sup.(a)                                                     0.21                                                                              0.42                                                                              0.51                                                                              0.68                                                                              0.57                                                                              2.06                                        Water Pore Volume, cc/g                                                                     0.26                                                                              0.36                                                                              0.38                                                                              0.37                                                                              0.44                                                                              0.65                                        Crush Strength, FPCS, lbs.sup.(b)                                                           20  15  21  28  15  No Data                                     Total Pore Volume, Hg, cc/g.sup.(c)                                                         1.26                                                                              0.42                                                                              0.40                                                                              0.40                                                                              0.42                                                                              0.65                                        Average Pore Diameter, Hg, Å.sup.(c)                                                    620 560 --  --  770 1000                                        Median Pore Diameter, Hg,                                                                   3.7 2.7 3.5 1.7 2.4 2.5                                         microns.sup.(c,e)                                                             Percent Pore Volume in Pores                                                                90.0%                                                                             88.5%                                                                             93.0%                                                                             --  91.5%                                                                             94.1%                                       Greater Than 350Å.sup.(c)                                                 Percent Pore Volume in Pores                                                                87.0%                                                                             82.5%                                                                             77.0%                                                                               --%                                                                             83.5%                                                                             61.0%                                       Greater Than 1 Micron.sup.(c)                                                 % Wt. Alpha Alumina                                                                         99.5                                                                              98  98.8                                                                              >98 98  70-75                                       Water Leachable Na, ppmw                                                                    12  53  69  --  18  No Data                                     Acid-Leachable Na, ppmw                                                                     40  96  188 172 45  No Data                                     Water Leachable K, ppmw                                                                     5   22  32  119 10  No Data                                     Acid-Leachable Fe, ppmw                                                                     2   5   --  --  5   No Data                                     % Wt. SiO.sub.2                                                                             .5  2   0.1 0.17                                                                              2   25-30                                       __________________________________________________________________________     .sup.(a) Method of Brunauer, Emmet and Teller, loc. cit.                      .sup.(b) Flat Plate Crush Strength, single pellet.                            .sup.(c) Determined by mercury intrusion to 55,000 psia using Micrometric     Autopore 9200 or 9210 (130° Contact angle, 0.473 N/m surface           tension of Hg).                                                               .sup.(e) Median pore diameter represents the pore diameter wherein 50% of     the total pore volume is found in pores having less than (or greater than     the median pore diameter.                                                

Of the carriers listed in TABLE I, C and D are preferred because theyprovide catalysts which have high initial selectivities.

The support, irrespective of the character of the support or carrierused, is preferably shaped into particles, chunks, pieces, pellets,rings, spheres, wagon wheels, and the like of a size suitable for use infixed bed reactors. Conventional commercial fixed bed reactors aretypically in the form of a plurality of parallel elongated tubes (in asuitable shell) approximately 0.7 to 2.7 inches O.D. and 0.5 to 2.5inches I.D. and 15-45 feet long filled with catalyst. In such reactors,it is desirable to use a support formed into a rounded shape, such as,for example, spheres, pellets, rings, tablets and the like, havingdiameters from about 0.1 inch to about 0.8 inch.

Particular supports having differing properties such as surface area andpore volume may be selected in order to provide particular catalyticproperties. With regard to surface area (B.E.T.), a possible lower limitis about 0.01 m² /g and a possible upper limit is about 10 m² /g. Withregard to water pore volume, a possible lower limit is about 0.05 cc/gand a possible upper limit is about 0.8 cc/g.

The catalysts of the present invention are prepared by a technique inwhich the alkali metal promoters, the rhenium, and the rheniumco-promoter, in the form of soluble salts and/or compounds are depositedon the catalyst and/or support prior to, simultaneously with, orsubsequent to the deposition of the silver and each other. The alkalimetals may be deposited at one step of the process and the rheniumand/or the rhenium co-promoter at a different step or steps. Thepreferred-method is to deposit silver, alkali metal, rhenium and rheniumco-promoter simultaneously on the support, that is, in a singleimpregnation step, although it is believed that the individual orconcurrent deposition of the alkali metal, rhenium and rheniumco-promoter prior to and/or subsequent to the deposition of the silverwould also produce suitable catalysts.

Promoting amounts of alkali metal or mixtures of alkali metal aredeposited on a porous support using a suitable solution. Although alkalimetals exist in a pure metallic state, they are not suitable for use inthat form. They are used as ions or compounds of alkali metals dissolvedin a suitable solvent for impregnation purposes. The carrier isimpregnated with a solution of alkali metal promoter ions, salt(s)and/or compound(s) before, during or after impregnation of the silverions or salt(s), complex(es), and/or compound(s) has taken place. Analkali metal promoter may even be deposited on the carrier afterreduction to metallic silver has taken place. The promoting amount ofalkali metal utilized will depend on several variables, such as, forexample, the surface area and pore structure and surface chemicalproperties of the carrier used, the silver content of the catalyst andthe particular ions used in conjunction with the alkali metal cation,the rhenium and the rhenium co-promoter, and the amounts of rhenium andrhenium co-promoter present. The amount of alkali metal promoterdeposited upon the support or present on the catalyst generally liesbetween about 10 parts per million and about 3000 parts per million,preferably between about 15 parts per million and about 2000 parts permillion and more preferably, between about 20 parts per million andabout 1500 parts per million by weight of total catalyst. Mostpreferably, the amount ranges between about 50 parts per million andabout 1000 parts per million by weight of the total catalyst. The degreeof benefit obtained within the above-defined limits will vary dependingupon particular properties and characteristics, such as, for example,reaction conditions, catalyst preparative techniques, surface area andpore structure and surface chemical properties of the carrier utilized,silver content of the catalyst, and other compounds, cations or anionspresent in addition to alkali metal ions such as ions added with thealkali metal, rhenium and rhenium co-promoter, or compounds remainingfrom the impregnating solution, and the above-defined limits wereselected to cover the widest possible variations in properties andcharacteristics. The effects of these variation in properties arereadily determined by experimentation. The alkali metal promoters arepresent on the catalysts in the form of cations (ions) or compounds ofcomplexes or surface compounds or surface complexes rather than as theextremely active free alkali metals, although for convenience purposesin this specification and claims they are referred to as "alkali metal"or "alkali metal promoters" even though they are not present on thecatalyst as metallic elements. For purposes of convenience, the amountof alkali metal deposited on the support or present on the catalyst isexpressed as the metal. Without intending to limit the scope of theinvention, it is believed that the alkali metal compounds are oxidiccompounds. More particularly, it is believed that the alkali metalcompounds are probably in the form of mixed surface oxides or doublesurface oxides or complex surface oxides with the aluminum of thesupport and/or the silver of the catalyst, possibly in combination withspecies contained in or formed from the reaction mixture, such as, forexample, chlorides or carbonates or residual species from theimpregnating solution(s).

In a preferred embodiment, at least a major proportion. (greater than50%) of the alkali metals comprise the higher alkali metals. As usedherein, the term "higher alkali metal" and cognates thereof refers tothe alkali metals selected from the group consisting of potassium,rubidium, cesium and mixtures thereof. As used herein, the term "alkalimetal" and cognates thereof refers to the alkali metals selected fromthe group consisting of lithium, sodium, potassium, rubidium, cesium andmixtures thereof. As used herein, the term "mixtures of alkali metals"or "mixtures of higher alkali metals" or cognates of these terms refersto the use of two or more of the alkali or higher alkali metals, asappropriate, to provide a promoting effect. Non-limiting examplesinclude cesium plus rubidium, cesium plus potassium, cesium plus sodium,cesium plus lithium, cesium plus rubidium plus sodium, cesium pluspotassium plus sodium, cesium plus lithium plus sodium, cesium plusrubidium plus potassium plus sodium, cesium plus rubidium plus potassiumplus lithium, cesium plus potassium plus lithium and the like. Apreferred alkali metal promoter is cesium. A particularly preferredalkali metal promoter is cesium plus at least one additional alkalimetal. The additional alkali metal is preferably selected from sodium,lithium and mixtures thereof, with lithium being preferred.

It should be understood that the amounts of alkali metal promoters onthe catalysts are not necessarily the total amounts of these metalspresent in the catalyst. Rather, they are the amounts of alkali metalpromoters which have been added to the catalyst by impregnation with asuitable solution of ions, salts and/or compounds and/or complexes ofalkali metals. These amounts do not include amounts of alkali metalswhich are locked into the support, for example, by calcining, or are notextractable in a suitable solvent such as water or lower alkanol oramine or mixtures thereof and do not provide a promoting effect. It isalso understood that a source oft he alkali metal promoter ions, saltsand/or compounds used to promote the catalyst may be the carrier. Thatis, the carrier may contain extractable amounts of alkali metal that canbe extracted with a suitable solvent, such as water or lower alkanol,thus preparing an impregnating solution from which the alkali metalions, salts and/or compounds are deposited or redeposited on thesupport.

As used herein, the term "compound" refers to the combination of aparticular element with one or more-different elements by surface and/orchemical bonding, such as ionic and/or covalent and/or coordinatebonding. The term "ionic" or "ion" refers to an electrically chargedchemical moiety; "cationic" or "cation" being positive and "anionic" or"anion" being negative. The term "oxyanionic" or "oxyanion" refers to anegatively charged moiety containing at least one oxygen atom incombination with another element. An oxyanion is thus anoxygen-containing anion. It is understood that ions do not exist invacuo, but are found in combination with charge-balancing counterions.The term"oxidic" refers to a charged or neutral species wherein anelement in question is bound to oxygen and possibly one or moredifferent elements by surface and/or chemical bonding, such as ionicand/or covalent and/or coordinate bonding. Thus, an oxidic compound isan oxygen-containing compound which also may be a mixed, double orcomplex surface oxide. Illustrative oxidic compounds include, by way ofnon-limiting examples, oxides (containing only oxygen as the secondelement), hydroxides, nitrates, sulfates, carboxylates, carbonates,bicarbonates, oxyhalides, etc. as well as surface species wherein theelement in question is bound directly or indirectly to an oxygen eitherin the substrate or the surface.

As used herein, the term "promoting amount" of a certain component of acatalyst refers to an amount of that component which works effectivelyto provide an improvement in one or more of the catalytic properties ofthat catalyst when compared to a catalyst not containing said component.Examples of catalytic properties include, inter alia, operability(resistance to runaway), selectivity, activity, conversion, stabilityand yield. It is understood by one skilled in the art that one or moreof the individual catalytic properties may be enhanced by the "promotingamount" while other catalytic properties may or may not be enhanced andmay even be diminished. It is further understood that differentcatalytic properties may be enhanced at different operation conditions.For example, a catalyst having enhanced selectivity at one set ofoperating conditions may be operated at a different set of conditionswherein the improvement shows up in the activity rather than theselectivity and an operator of an ethylene oxide plant willintentionally change the operation conditions in order to take advantageof certain catalytic properties even at the expense of other catalyticproperties in order to maximize profits by taking into account feedstockcosts, energy costs, by-product removal costs and the like. Theparticular combination of silver, support, alkali metal promoter,rhenium. promoter and rhenium co-promoter of the instant invention willprovide an improvement in one or more catalytic properties over the samecombination of silver, support, alkali metal promoter, rhenium promoterand no rhenium co-promoter.

As used herein, the term "catalytically effective amount of silver"refers to an amount of silver that provides a measurable conversion ofethylene and oxygen to ethylene oxide.

The carrier is also impregnated with rhenium ions, salt(s), compound(s),and/or-complex(es). This may be done at the same time that the alkalimetal promoter is added, or before or later; or at the same time thatthe rhenium co-promoter is added, or before or later. Preferably,rhenium, alkali metal, rhenium co-promoter and silver are in the sameimpregnating solution, although it is believed that their presence indifferent solutions will still provide suitable catalysts. The preferredamount of rhenium, calculated as the metal, deposited on or present onthe carrier or catalyst ranges from about 0.1 micromoles per gram toabout 10 micromoles per gram, more preferably from about 0.2 micromolesper gram to about 5 micromoles per gram of total catalyst, or,alternatively stated, from about 19 parts per million to about 1860parts per million, preferably from about 37 parts per million to about930 parts per million by weight of total catalyst. The degree of benefitobtained within the above-defined limits will vary depending uponparticular properties and characteristics, such as, for example,reaction conditions, catalyst preparative conditions, surface area andpore structure and surface chemical properties of the carrier utilized,silver content of the catalyst and other compounds, anions or cationspresent in addition to those containing rhenium, alkali metal, orrhenium co-promoter, such as ions added with the alkali metal, rheniumor rhenium co-promoter, or compounds remaining from the impregnatingtechnique, and the above-defined limits were selected to cover thewidest possible variations in properties and characteristics. Theeffects of these variations are readily determined by experimentation.For purposes of convenience, the amount of rhenium present on thecatalyst is expressed as the metal, irrespective of the form in which itis present.

The rhenium compounds used in the preparation of the instant catalystsare rhenium compounds that can be solubilized in an appropriate solvent.Preferably, the solvent is a water-containing solvent. More preferably,the solvent is the same solvent used to deposit the silver and thealkali metal promoter. Examples of suitable rhenium compounds includethe rhenium salts such as rhenium halides, the rhenium oxyhalides, therhenates, the perrhenates, the oxides and the acids of rhenium. Apreferred compound for use in the impregnation solution is theperrhenate, preferably ammonium perrhenate. However, the alkali metalperrhenates, alkaline earth metal perrhenates, silver perrhenates, otherperrhenates and rhenium heptoxide can also be suitably utilized. Rheniumheptoxide, Re₂ O₇, when dissolved in water, hydrolyzes to perrhenicacid, HReO₄, or hydrogen perrhenate. Thus, for purposes of thisspecification, rhenium heptoxide can be considered to be a perrhenate,i.e., ReO₄. It is also understood that there are many rhenium compoundsthat are not soluble per se in water. However, these compounds can besolubilized by utilizing various acids, bases, peroxides, alcohols, andthe like. After solubilization, these compounds could be used, forexample, with an appropriate amount of water or other suitable solventto impregnate the carriers. Of course, it is also understood that uponsolubilization of many of these compounds, the original compound nolonger exists after solubilization. For example, rhenium metal is notsoluble in water. However, it is soluble in concentrated nitric acid aswell as in hydrogen peroxide solution. Thus, by using an appropriatereactive solvent, one could use rhenium metal to prepare a solubilizedrhenium-containing impregnating solution. In a preferred embodiment ofthe instant invention, the rhenium present on the catalyst is present ina form that is extractable in a dilute aqueous base solution.

The carrier is also impregnated with a rhenium co-promoter selected fromthe group consisting of phosphorus, boron and mixtures thereof. It hasbeen found that adding a rhenium co-promoter selected from phosphorus,boron and mixtures thereof to an alkali metal/rhenium doped supportedsilver catalyst results in an improvement in initial selectivity of thecatalyst. The exact form of the phosphorus and/or boron co-promoter onthe catalyst is not known. The co-promoter, it is believed, is notpresent on the catalyst in the elemental form since the co-promoter isapplied to catalyst in the form of ions, salts, compounds and/orcomplexes and the reducing conditions generally used to reduce thesilver to metallic silver are not usually sufficient to reduce thephosphorus or boron to the elemental form. It is believed that theco-promoter deposited on the support or present on the catalyst is inthe compound form, and probably in the form of an oxygen-containing oroxidic compound. In a presently preferred embodiment, the co-promoter isapplied to the catalyst in the oxyanionic form, i.e, in the form of ananion, or negative ion which contains oxygen. Examples of anions ofphosphorus that can be suitably applied include phosphate, i.e.,ortho-phosphate, hydrogen phosphate, dihydrogen phosphate,metaphosphate, fluorophosphate, pyrophosphate, hypophosphate,diphosphate, triphosphate, etc. Examples of anions of boron that cansuitably be applied include borate, metaborate, tetraborate,tetrafluoroborate, etc. The anions can be supplied with variouscounter-ions. Preferred are ammonium, alkali metal, mixed alkali metaland hydrogen (i.e. acid form). The anions can be prepared by thereactive dissolution of various non-anionic materials such as the oxidessuch as P₂ O₅, B₂ O₃, etc., as well as other materials such as halides,oxyhalides, hydroxyhalides, hydroxides, sulfides, etc., of the metals.

The carrier may be impregnated with rhenium co-promoter ions, salt(s),compound(s) and/or complex(es) at the same time that the othercomponents are added, or before and/or later. Preferably, rheniumco-promoter, rhenium, alkali metal and silver are in the sameimpregnating solution, although it is believed that their presence indifferent solutions will still provide suitable catalysts.

The preferred amount of co-promoter compound present on or deposited onthe support or catalyst ranges from about 0.1 to about 10 micromoles,preferably from about 0.2 to about 5 micromoles, expressed as theelement, per gram of total catalyst. The degree of benefit obtainedwithin the above-defined limits will vary depending upon particularproperties and characteristics, such as, for example, reactionconditions, catalyst preparative techniques, surface area and porestructure and surface chemical properties of the carrier utilized,silver content of the catalyst, alkali content of the catalyst, rheniumcontent of the catalyst, and other compounds anions or cations presentbesides those containing rhenium, rhenium co-promoter and alkali metal,such as ions added with the alkali metal, rhenium or rheniumco-promoter, or compounds remaining from the impregnation technique, andthe above-defined limits were selected to cover the widest possiblevariations in properties and characteristics. These variations arereadily determined by experimentation. For purposes of convenience theamount of co-promoter present on the catalyst is expressed as theelement irrespective of the form in which it is present.

The presence of the indicated and claimed promoting amount of rheniumco-promoters in this specification and claims does not preclude the useof other activators, promoters, enhancers, stabilizers, improvers, etc.,and it is not intended by the use of Markush terminology in thisspecification and claims to exclude the use of such other activators,promoters, enhancers, stabilizers, improvers, etc.

The co-promoter compounds, salts and/or complexes suitable for use inthe preparation of the instant catalysts are compounds, salts and/orcomplexes which can be solubilized in an appropriate solvent.Preferably, the solvent is a water-containing solvent. More preferably,the solvent is the same solvent used to deposit the silver, alkali metalpromoter and rhenium. Preferred co-promoter compounds are the oxyanioniccompounds of the co-promoter elements, preferably the ammonium andalkali metal oxyanionates, such as ammonium phosphate, potassiumphosphate, lithium phosphate, sodium phosphate, potassium hydrogenphosphate, potassium dihydrogen phosphate, potassium borate, lithiumborate, sodium borate, potassium metaborate, lithium metaborate, sodiummetaborate, potassium tetraborate, sodium tetraborate, lithiumtetraborate and the like.

Generally, the carrier is contacted with a silver salt, a silvercompound, or a silver complex which has been dissolved in an aqueoussolution, so that the carrier is impregnated with said aqueous solution;thereafter the impregnated carrier is separated .form the aqueoussolution, e.g., by centrifugation or filtration and then dried. The thusobtained impregnated carrier is heated to reduce the silver to metallicsilver. It is conveniently heated to a temperature in the range of fromabout 50° C. to about 600° C., during a period sufficient to causereduction of the silver salt, compound or complex to metallic silver andto form a layer of finely divided silver, which is bound to the surfaceof the carrier, both the exterior and pore surface. Air,. or otheroxidizing gas, reducing gas, an inert gas or mixtures thereof may beconducted over the carrier during this heating step.

There are several known methods to add the silver to the carrier orsupport. The carrier may be impregnated with an aqueous solutioncontaining silver nitrate dissolved therein, and then dried, after whichdrying step the silver nitrate is reduced with hydrogen or hydrazine.The carrier may also be impregnated with an ammoniacal solution ofsilver oxalate or silver carbonate, and then dried, after which dryingstep the silver oxalate or silver carbonate is reduced to metallicsilver by heating, e.g., to about 600° C. Specific solutions of silversalts with solubilizing and reducing agents may be employed as well,e.g., combinations of the vicinal alkanolamtnes, alkyldiamines andammonia. One such example of a solution of silver salts comprises animpregnating solution comprising a silver salt of a carboxylic acid, anorganic amine alkaline solubilizing/reducing agent, and an aqueoussolvent.

Suitable carboxylic acid silver salts include silver carbonate and thesilver salts of mono- and polybasic carboxylic and hydrocarboxylic acidsof up to about 16 carbon atoms. Silver carbonate and silver oxalate areparticularly useful silver salts, with silver oxalate being mostpreferred.

An organic amine solubilizing/reducing agent is present in theimpregnating solution. Suitable organic aminesilver-solubilizing/reducing agents include lower alkylenediamines offrom 1 to 5 carbon atoms, mixtures of a lower alkanolamine of from 1 to5 carbon atoms with a lower alkylenediamine of from 1 to 5 carbon atoms,as well as mixtures of ammonia with lower alkanolamines or loweralkylenediamines for from 1 to 5 carbons. Four groups of organic aminesolubilizing/reducing agents are particularly useful. The four groupsinclude vicinal alkylenediamines of from 2 to 4 carbon atoms, mixturesof (1) vicinal alkanolamines of from 2 to 4 carbon atoms and (2) vicinalalkylenediamines of from 2 to 4 carbon atoms, mixtures of vicinalalkylenediamines of from 2 to 4 carbon atoms and ammonia, and mixturesof vicinal alkanolamines of from 2 to 4 carbon atoms and ammonia. Thesesolubilizing/reducing agents are generally added in the amount of fromabout 0.1 to about 10 moles per mole of silver present.

Particularly preferred solubilizing/reducing agents are ethylenediamine,ethylenediamine in combination with ethanolamine, ethylenediamine incombination with ammonia, and ethanolamine in combination with ammonia,with ethylenediamine being most preferred. Ethylenediamine incombination with ethanolamine gives comparable results, but it isbelieved that impurities present in certain commercially availableethanolamine preparations can produce inconsistent results.

When ethylenediamine is used as the sole solubilizing/reducing agent, itis necessary to add amount of the amine in the range of from about 0.1to about 5.0 moles of ethylenediamine per mole of silver.

When ethylenediamine and ethanolamine together are used as thesolubilizing/reducing agent, it is suitable to employ from about 0.1 toabout 3.0 moles of ethylenediamine per mole of silver and from about 0.1to about 2.0 moles of ethanolamine per mole of silver.

When ethylenediamine or ethanolamine is used with ammonia, it isgenerally useful to add at least about two moles of ammonia per mole ofsilver and very suitable to add from about 2 to about 10 moles ofammonia per mole of silver. The amount of ethylenediamine orethanolamine employed then is suitably from 0.1 to 2.0 moles per mole ofsilver.

One method of preparing the silver containing catalyst can be found inU.S. Pat. No. 3,702,259, issued Nov. 7, 1972, incorporated by referenceherein. Other methods for preparing the silver-containing catalystswhich in addition contain higher alkali metal promoters can be found inU.S. Pat. No. 4,010,115, issued Mar. 1, 1977; and U.S. Pat. No.4,356,312, issued Oct. 26, 1982; U.S. Pat. No. 3,962,136, issued Jun. 8,1976 and U.S. Pat. No. 4,012,425, issued Mar. 15, 1977, all incorporatedby reference herein. Methods for preparing silver-containing catalystscontaining higher alkali metal and rhenium promoters can be found inU.S. Pat. No. 4,761,394, issued Aug. 2, 1988, which is incorporated byreference herein, and methods for silver-containing catalysts containinghigher alkali metal and rhenium promoters and sulfur and/or molybdenumand/or tungsten and/or chromium rhenium co-promoters can be found inU.S. Pat. No. 4,766,105, issued Aug. 2, 1988, which is incorporatedherein be reference.

The preferred amount of alkali metal promoter deposited on or present onthe surface of the carrier of catalyst generally lies between about 10parts per million and about 3000 parts per million, preferably betweenabout 15 parts per million and about 2000 parts per million and morepreferably between about 20 parts per million and about 1500 parts permillion by weight of alkali metal .calculated on the total carriermaterial. Amounts between about 50 parts per million and about 1000parts per million are most preferable. Suitable compounds of alkalimetals comprise lithium, sodium, potassium, rubidium, cesium or mixturesthereof in a promoting amount with the even more preferred promotersbeing rubidium and/or cesium plus an additional alkali metal selectedfrom lithium, sodium and mixtures thereof. Preferably the amount rangesfrom about 10 parts per million and about 3000 parts per million, morepreferably between about 15 parts per million and about 2000 parts permillion, even more preferably between about 20 parts per million andabout 1500 parts per million by weight, and most preferably betweenabout 50 parts per million and 1000 parts per million by weight. Themost preferred promoter is cesium plus lithium, preferably applied in anaqueous solution having cesium nitrate or cesium hydroxide dissolvedtherein.

There are known excellent methods of applying the promoterscoincidentally with the silver on the carrier. Suitable alkali metalsalts are generally those which are soluble in the silver-impregnatingliquid phase. Besides the above-mentioned compounds may be mentioned thenitrites; the halides, such as fluorides, chlorides, iodides, bromides;oxyhalides; bicarbonates; borates; sulfates; sulfites; bisulfates;acetates; tartrates; lactates and isopropoxides, etc. The use of alkalimetal, rhenium or co-promoter salts which have ions which react with thesilver salt in solution is preferably avoided, e.g. the use of cesiumchloride together with silver nitrate in an aqueous solution, since thensome silver chloride is prematurely precipitated. Here the use of cesiumnitrate is recommended instead of cesium chloride, for example. However,cesium chloride may be used together with a silver salt-amine-complex inaqueous solution, since then the silver chloride is not precipitatedprematurely from the solution.

The promoters may be deposited on the carrier (support) or on thecatalyst, depending upon the particular impregnation technique orsequence utilized. In this specification and claims, the term "on thecatalyst" when referring to the deposition or presence of promotersand/or co-promoters refers to the catalyst which comprises thecombination of carrier (support) and silver. Thus, the promoters, i.e.,alkali metal, rhenium and rhenium co-promoter may be found individuallyor in a mixture thereof on the catalyst, on the support or on both thecatalyst and the support. There may be, for example, alkali, rhenium andrhenium co-promoter on the support; alkali metal, rhenium and rheniumco-promoter on the catalyst; alkali metal, and rhenium on the supportand rhenium co-promoter on the catalyst; alkali metal, and rheniumco-promoter on the support and rhenium on the catalyst; alkali metal,rhenium and rhenium co-promoter on the support and rhenium and rheniumco-promoter on the catalyst and any of the other possible distributionsof alkali metal, rhenium and/or rhenium co-promoter between the supportand/or the catalyst.

The amount of the alkali metal and rhenium promoters and rheniumco-promoters on the porous carrier or catalyst may also be regulatedwithin certain limits by washing out the surplus of promoter materialwith an appropriate solvent, for example, methanol or ethanol.

A particularly preferred process of impregnating the carrier consists ofimpregnating the carrier with an aqueous solution containing a silversalt of a carboxylic acid, an organic amine, a salt of cesium, ammoniumperrhenate and ammonium sulfate dissolved therein. Silver oxalate is apreferred salt. It can be prepared by reacting silver oxide (slurry inwater) with (a) a mixture of ethylenediamine and oxalic acid, or (b)oxalic acid and then ethylenediamine, which latter is preferred, so thatan aqueous solution of silver oxalate-ethylenediamine complex isobtained, to which solution is added a certain amount of cesiumcompound, ammonium perrhenate and ammonium sulfate. While addition ofthe amine to the silver oxide before adding the oxalic acid is possible,it is not preferred since it can give rise to solutions which areunstable or even explosive in nature. Other diamines and other amines,such as ethanolamine, may be added as well. A cesium-containing silveroxalate solution may also be prepared by precipitating silver oxalatefrom a solution of cesium oxalate and silver nitrate and rinsing withwater or alcohol the obtained silver oxalate in order to remove theadhering cesium salt until the desired cesium content is obtained. Thecesium-containing silver oxalate is then solubilized with ammonia and/oran amine in water and ammonium perrhenate and ammonium sulfate is added.Rubidium-, potassium-, sodium-, lithium- and mixtures of alkalimetal-containing solutions may be prepared also in these ways. Theimpregnated carriers are then heated to a temperature between about 50°C. and about 600° C., preferably between about 75° C. and about 400° C.to evaporate the liquid and produce a metallic silver.

In general terms, the impregnation process comprises impregnating thesupport with one or more solutions comprising silver, alkali metal,rhenium and rhenium co-promoter. As used in the instant specificationand claims, the terminology "impregnating the support with one or moresolutions comprising silver, alkali metal, rhenium and rheniumco-promoter" and similar or cognate terminology means that the supportis impregnated in a single or multiple impregnation with one solutioncontaining silver, alkali metal, rhenium and rhenium co-promoter indiffering amounts; or in multiple impregnations with two or moresolutions, wherein each solution contains at least one componentselected from silver, alkali metal, rhenium and rhenium co-promoter,with the proviso that all of the components of silver, alkali metal,rhenium and rhenium co-promoter will individually be found in at leastone of the solutions. The concentration of the silver (expressed as themetal) in the silver-containing solution will range from about 1 g/l upto the solubility limit when a single impregnation is utilized. Theconcentration of the alkali metal (expressed as the metal) will rangefrom about 1×10⁻³ g/l up to about 12 g/l and preferably, from about10×10⁻³ g/l to about 12 g/l when a single impregnation step is utilized.The concentration of the rhenium (expressed as the metal) will rangefrom about 5×10⁻³ g/l to about 20 g/l and preferably from about 50×10⁻³g/l to about 20 g/l when a single impregnation step is utilized. Theconcentration of rhenium co-promoter (expressed as the element) willrange from about 1×10⁻³ g/l to about 20 g/l and preferably from about10×10⁻³ g/l to about 20 g/l when a single impregnation step is utilized.Concentrations selected within the above noted ranges will depend uponthe pore volume of the catalyst, the final amount desired in the finalcatalyst and whether the impregnation is single or multiple. Appropriateconcentrations can be readily determined by routine experimentation.

The amount of silver deposited on the support or present on the supportis to be a catalytically effective amount of silver, i.e., an amountthat catalyzes the reaction of ethylene and oxygen to produce ethyleneoxide. Preferably this amount will range from about 1 to about 30percent by weight of the total catalyst, more preferably from about 1 toabout 25 percent by weight of the total catalyst, and even morepreferably from about 5 to about 20 percent by weight of the totalcatalyst. The upper and lower limits of preferred silver concentrationscan be suitably varied, depending upon the particular catalyticproperties or effect desired or the other variables involved. The lowerlimit of silver is probably about 1 percent by weight of the totalcatalyst, and the upper limit of silver is probably about 30 percent byweight of the total catalyst.

The amount of alkali metal deposited on the support or catalyst orpresent on the support or catalyst is to be a promoting amount.Preferably the amount will range from about 10 parts per million toabout 3000 parts per million, more preferably from about 15 parts permillion to about 2000 parts per million and even more preferably fromabout 20 parts per million to about 1500 parts per million, and mostpreferably, from about 50 parts per million to about 1000 parts permillion by weight of the total catalyst, expressed as the metal. Theupper and lower limits of preferred alkali metal concentrations can besuitably varied depending upon the particular promoting effect desiredor other variables involved. A possible lower limit of alkali metal is,for example, about 1 parts per million by weight of the total catalyst,expressed as the metal, and a possible upper limit of alkali metal is,for example, about 3000 parts per million by weight of the totalcatalyst, expressed as the metal.

The amount of rhenium deposited on the support or catalyst or present onthe support of catalyst is to be a promoting amount. Preferably, theamount will range from about 0.1 micromoles per gram to about 10micromoles per gram, preferably from about 0.2 micromoles per gram toabout 5 micromoles per gram, and more preferably, from about 0.5micromoles per gram to about 4 micromoles per gram of total catalyst,expressed as the metal. The upper and lower limits of preferred rheniumconcentrations can be suitably varied depending upon the particularpromoting effect desired or other variables involved. A possible lowerlimit of rhenium is, for example, about 0.01 micromoles per gram oftotal catalyst, and a possible upper limit of rhenium is, for example,about 16 micromoles per gram of total catalysts expressed as the metal.

The amount of rhenium co-promoter deposited on the support or catalystor present on the support or catalyst is to be a promoting amount.Preferably, the amount will range from about 0.1 micromoles per gram toabout 10 micromoles per gram, preferably from about 0.2 micromoles pergram to about 5 micromoles per gram, and more preferably, from about 0.5micromoles per gram to about 4 micromoles per gram of total catalyst,expressed as the element or the metal. The upper and lower limits ofpreferred rhenium co-promoter concentrations can be suitably varieddepending upon the particular promoting effect desired or othervariables involved. A possible lower limit of rhenium co-promoter is,for example, about 0.01 micromoles/gram of total catalyst, and apossible upper limit of rhenium co-promoter is, for example, about 16micromoles/gram of total catalyst, measured as the metal.

It is observed that independent of the form in which the silver ispresent in the solution before precipitation on the carrier, the term"reduction to metallic silver" is used, while in the meantime oftendecomposition by heating occurs. We prefer to use the term "reduction",since Ag⁺ ion is converted into a metallic Ag atom. Reduction times maygenerally vary from about 0.5 minute to about 8 hours, depending on thecircumstances.

The silver catalysts according to the present invention have been shownto have a particularly high initial selectivity for ethylene oxideproduction in the direct oxidation of ethylene with molecular oxygen toethylene oxide. The conditions for carrying out such an oxidationreaction in the presence of the silver catalysts according to thepresent invention broadly comprise those already described in the priorart. This applies, for example, to suitable temperatures, pressures,residence times, diluent materials such as nitrogen, carbon dioxide,steam, argon, methane or other saturated hydrocarbons, to the presenceof moderating agents to control the catalytic action, for example,1-2-dichloroethane, vinyl chloride, ethyl chloride or chlorinatedpolyphenyl compounds, to the desirability of employing recycleoperations or applying successive conversations in different reactors toincrease the yields of ethylene oxide, and to any other specialconditions which may be selected in processes for preparing ethyleneoxide. Pressures in the range of from atmospheric to about 500 psig aregenerally employed. Higher pressures, however, are not excluded.Molecular oxygen employed as reactant can be obtained from conventionalsources. The suitable oxygen charge may consist essentially orrelatively pure oxygen, a concentrated oxygen stream comprising oxygenin major amount with lesser amounts of one or more diluents, such asnitrogen and argon, or another oxygen-containing stream, such as air. Itis therefore evident that the use of the present silver catalysts inethylene oxide reactions is in no way limited to the use of specificconditions among those which are known to be effective. For purposes ofillustration only, the following table shows the range of conditionsthat are often used in current commercial ethylene oxide reactor units.

                  TABLE II                                                        ______________________________________                                        *GHSV                 1500-10,000                                             Inlet Pressure         150-400 psig                                           Inlet Feed                                                                    Ethylene                1-40%                                                 O.sub.2                 3-12%                                                 Ethane                  0-3%                                                  Argon and/or methane and/or nitrogen                                                                  0.3-20 ppmv total                                     diluent chlorohydrocarbon moderator                                           Coolant temperature    180-315° C.                                     Catalyst temperature   180-325° C.                                     O.sub.2 conversion level                                                                             10-60%                                                 EO Production (Work Rate)                                                                             2-16 lbs. EO/cu.                                                            ft. catalyst/hr.                                        ______________________________________                                         *Cubic feet of gas at standard temperature and pressure passing over one      cubic foot of packed catalyst per hour.                                  

In a preferred application of the silver catalysts according to thepresent invention, ethylene oxide is produced when an oxygen-containinggas is contacted with ethylene in the presence of the present catalystsat a temperature in the range of from about 180° C. to about 330° C. andpreferably about 200° C. to about 325° C.

The ranges and limitations provided in the instant specification andclaims are those which are believed to particularly point out anddistinctly claim the present invention. It is, however, understood thatother ranges and limitations which perform substantially the samefunction in the same or substantially the same manner to obtain the sameor substantially the same result are intended to be within the scope ofthe instant invention as defined by the instant specification andclaims.

The invention will be illustrated by the following illustrativeembodiments which are provided for illustration only and are notintended to limit the scope of the instant invention.

ILLUSTRATIVE EMBODIMENTS Illustrative Embodiment 1

The following illustrative embodiment describes typical preparativetechniques for making the catalysts of the instant invention (andcomparative catalysts) and the typical technique for measuring theproperties of these catalysts.

Part A: Preparation of stock silver oxalate/ethylene-diamine solutionfor use in catalyst preparation:

1) Dissolve 415 grams (g) of reagent-grade sodium hydroxide in 2340milliliters (ml) deionized water. Adjust the temperature to 50° C.

2) Dissolve 1699 g of "Spectropure" (high purity) silver nitrate in 2100ml deionized water. Adjust the temperature to 50° C.

3) Add sodium hydroxide solution slowly to silver nitrate solution withstirring while maintaining a temperature of 50° C. Stir for 15 minutesafter addition is complete, and then lower the temperature to 40° C.

4) Insert clean filter wands and withdraw as much water as possible fromthe precipitate created in step (3) in order to remove sodium andnitrate ions. Measure the conductivity of the water removed and add backas much fresh deionized water as was removed by the filter wands. Stirfor 15 minutes at 40° C. Repeat this process until the conductivity ofthe water removed is less than 90 μmho/cm. Then add back 1500 mldeionized water.

5) Add 630 g of high-purity oxalic acid dihydrate in approximately 100 gincrements. Keep the temperature at 40° C. and stir to mix thoroughly.Add the last portion of oxalic acid dihydrate slowly and monitor pH toensure that pH does not drop below 7.8.

6) Remove as much water from the mixture as possible using clean filterwands in order to form a highly concentrated silver-containing slurry.Cool the silver oxalate slurry to 30° C.

7) Add 699 g of 92 percent weight (%w) ethylenediamine (8% deionizedwater). Do not allow the temperature to exceed 30° C. during addition.

The above procedure yields a solution containing approximately 27-33%wsilver.

Part B: Preparation of impregnation solutions

For Catalyst A (1.5 Re/0.33 P), into a 10 ml beaker is added 0.163 g of(NH₄)ReO₄ and approximately 2.0 g of ethylene-diamine/H₂ O (50/50 byweight), and the mixture is allowed to dissolve with stirring. 0.0157 gof (NH₄)H₂ PO₄ is dissolved in 1 ml of water in a weighing dish, andthen added to the perrhenate solution. 0.3422 g of LiNO₃ is dissolved in2 ml of water and added to the perrhenate solution. Theperrhenate/phosphate/nitrate solution is allowed to stir, ensuringcomplete dissolution. This solution are then added to 183 g of theabove-prepared silver solution (specific gravity=1.556 g/cc), and theresulting solution is diluted with water to a total weight of 204 g.One-fourth of this solution is used to prepare a catalyst. 0.082 g ofCsOH solution (42.9% Cs) is added to a 51 g portion of the silveroxalate/dopant solution to prepare the final impregnation solution.

For Catalyst B (1.5 Re/1.0 P), the procedure for Catalyst A is followed,except that 0.0476 g of (NH₄)H₂ PO₄ is used.

For Catalyst C (1.5 Re/0.33 B), the procedure for Catalyst A isfollowed, except that 0.0084 g of H₃ BO₃ is used in place of the (NH₄)H₂PO₄.

For Catalyst D, (1.5 Re/4.0 B), the procedure for Catalyst A isfollowed, except that 0.1018 g of H₃ BO₃ is used in place of the (NH₄)H₂PO₄ and 0.1013 g of the aqueous CsOH solution is used.

For Catalyst E (2.0 Re), the general procedure for catalyst A isfollowed. However, no phosphate or borate co-promoter is added. 0.055 gof (NH₄)ReO₄ is dissolved in a minimum of 50/50 by weightethylenediamine/H₂ O solution, and this is added to 50 g of theabove-prepared silver solution (specific gravity=1.556 g/co). 0.0847 gof CsOH solution (50.7% Cs) is also added to the impregnation solution.

Part C: Catalyst impregnation and curing

For Catalysts A and B, approximately 30 g of carrier C (described inTable 1) are placed under 25 mm vacuum for 3 minutes at roomtemperature. Approximately 50 g of doped impregnating solution is thenintroduced to submerge the carrier, and the vacuum is maintained at 25mm for an additional 3 minutes. At the end of this time, the vacuum isreleased, and excess impregnating solution is removed from the carrierby centrifugation for 2 minutes at 500 rpm. If the impregnating solutionis prepared without monoethanolamine, then the impregnated carrier isthen cured by being continuously shaken in a 300 cu. ft/hr. air streamflowing across a cross-sectional area of approximately 3-5 square inchesat 250°-270° C. for 5-6 minutes. If significant monoethanolamine ispresent in the impregnating solution, then the impregnated carrier iscured by being continuously shaken in a 300 cu. ft./hr. air stream at250° C. for 2.5 minutes, followed by a 100 cu. ft./hr. air stream at270° C. for 7.5 minutes (all over a cross-section area of approximately3-5 square inches). The cured catalyst is then ready for testing.

For Catalysts C and D, the above procedure is followed, except carrier D(described in Table 1) is used as the support.

For Catalyst E, the above procedure is followed, except carrier B(described in Table 1) is used as the support.

This procedure will yield catalysts on this carrier which containapproximately 13.5%w Ag with the following approximate dopant levels(expressed in parts per million by weight basis the weight of the totalcatalyst, i.e., ppmw) and which are approximately optimum in cesium forthe given silver and rhenium and co-promoter levels (P, B) and supportwith regard to initial selectivity under the test conditions describedbelow.

    ______________________________________                                               Cs, ppmw                                                                             Re, ppmw   P, ppmw  B, ppmw                                     ______________________________________                                        Catalyst A                                                                             340      279        10     --                                        Catalyst B                                                                             340      279        31     --                                        Catalyst C                                                                             340      279        --      4                                        Catalyst D                                                                             460      279        --     43                                        Catalyst E                                                                             438      372        --     --                                        ______________________________________                                    

The actual silver content of the catalyst can be determined by any of anumber of standard, published procedures. The actual level of rhenium onthe catalysts prepared by the above process can be determined byextraction with 20mM aqueous sodium hydroxide, followed byspectrophotometric determination of the rhenium in the extract. Theactual level of cesium on the catalyst can be determined by employing astock cesium hydroxide solution, which has been labeled with aradioactive isotope of cesium, in catalyst preparation. The cesiumcontent of the catalyst can then be determined by measuring theradioactivity of the catalyst. Alternatively, the cesium content of thecatalyst can be determined by leaching the catalyst with boilingdeionized water. In this extraction process cesium, as well as otheralkali metals, is measured by extraction from the catalyst by boiling 10grams of whole catalyst in 20 milliliters of water for 5 minutes,repeating the above two more times, combining the above extractions anddetermining the amount of alkali metal present by comparison to standardsolutions of reference alkali metals using atomic absorptionspectroscopy (using Perkin Elmer Model 1100 B). It should be noted thatthe cesium content of the catalyst as determined by the water leachingtechnique may be lower than the cesium content of the catalyst asdetermined by the radiotracer technique.

Part D: Standard Microreactor Catalyst Test Conditions/Procedure

3 to 5 grams of crushed catalyst (14-20 mesh) are loaded into a 1/4 inchdiameter stainless steel U-shaped tube. The U tube is immersed in amolten metal bath (heat medium) and the ends are connected to a gas flowsystem. The weight of the catalyst used and the inlet gas flow rate areadjusted to achieve a gas hourly space velocity of 3300 cc of gas per ccof catalyst per hour. The inlet gas pressure is 210 psig.

The gas mixture passed thorough the catalyst bed (in once-throughoperation) during the entire test run (including startup) consists of30% ethylene, 8.5% oxygen, 5% carbon dioxide, 54.5% nitrogen, and 2.0 to6.0 ppmv ethyl chloride.

The initial reactor (heat medium) temperature is 225° C. After 1 hour atthis initial temperature, the temperature is increased to 235° C. for 1hour, and then adjusted to 245° C. for 1 hour. The temperature is thenadjusted so as to achieve a constant oxygen conversion level of 40%.Performance data at this conversion level are usually obtained when thecatalyst has been onstream for a total of at least 2-3 days. Due toslight differences in feed gas composition, gas flow rates, and thecalibration of analytical instruments used to determine the feed andproduct gas compositions, the measured selectivity and activity of agiven catalyst may vary slightly from one test run to the next. To allowmeaningful comparison of the performance of catalysts tested atdifferent times, all catalysts described in this illustrative embodimentwere tested simultaneously with a reference catalyst. All performancedata reported in this illustrative embodiment are corrected relative tothe average initial performance of the reference catalyst (S₄₀ =81.0%;T₄₀ =230° C.).

The results are presented below in Table III. All selectivity values areexpressed as %.

                  TABLE III                                                       ______________________________________                                        Initial Performance of Catalyst                                               Catalyst       Initial Selectivity                                            ______________________________________                                        A (1.5Re/0.33P)                                                                              85.9%                                                          B (1.5Re/1.0P) 85.0%                                                          C (1.5Re/0.33B)                                                                              85.4%                                                          D (1.5Re/4.0B) 85.6%                                                          E (2.0Re)      81.9%                                                          ______________________________________                                    

As mentioned previously, selectivity is of tremendous importance andhigher selectivities are desired in order to run the process in a moreefficient and cost effective manner. As can be seen in Table III, boththe catalysts which contain a phosphorus co-promoter in combination withrhenium (Catalysts A and B), and the catalysts which contain a boronco-promoter in combination with rhenium (Catalysts B and C), have higherinitial selectivities than the catalyst which contains no phosphorus orboron rhenium co-promoter (Catalyst E).

What is claimed is:
 1. In a process for the production of ethylene oxidewherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature in the range of from about 180° C. to about 330° C. in thepresence of a silver metal-containing catalyst, the improvement whichcomprises using a catalyst comprising a catalytically effective amountof silver, a promoting amount of alkali metal, a promoting amount ofrhenium and a rhenium co-promoter selected from phosphorus, boron andmixtures thereof supported on a suitable support having a surface areain the range of from about 0.05m² /g to about 10 m² /g.
 2. In a processfor the production of ethylene oxide wherein ethylene is contacted inthe vapor phase with an oxygen-containing gas at ethylene oxide formingconditions at a temperature in the range of from about 180° C. to about330° C. in the presence of a silver metal-containing catalyst, theimprovement which comprises using a catalyst comprising from about 1percent by weight to about 25 percent by weight of the total catalyst ofthe silver compound(s), expressed as the metal, from about 10 parts permillion to about 3000 parts per million by weight of alkali metalcompound(s), expressed as the metal, per million parts by weight of thetotal catalyst, from about 0.1 micromoles to about 10 micromoles pergram of total catalyst of rhenium compound(s), expressed as the metal,and from about 0.1 micromoles to about 10 micromoles per gram of totalcatalyst of rhenium co-promoter compound(s) wherein the rheniumco-promoter compound is selected from compounds of phosphorus, boron andmixtures thereof, expressed as the element, to provide the catalyst witha catalytically effective amount of silver, a promoting amount of alkalimetal, a promoting amount of rhenium and a promoting amount of rheniumco-promoter.